feat: [CPT] I can read how to write process tests

[saig0]

Description

Adds a new reference documentation for the new Camunda Process Test Java library. It describes how to install and use the library to write process tests. Take inspiration from the Zeebe Process Test documentation.

The new documentation is added to the sidebar under: API & Tools > Testing > Camunda Process Test.

closes #3921

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).
• I included my new page in the sidebar file(s).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.

[github-actions]

👋 🤖 🤔 Hello! Did you make your changes in all the right places?

These files were changed only in docs/. You might want to duplicate these changes in versioned_docs/version-8.5/.

• docs/apis-tools/testing/assertions.md
• docs/apis-tools/testing/connectors.md
• docs/apis-tools/testing/getting-started.md
• docs/apis-tools/testing/utilities.md

You may have done this intentionally, but we wanted to point it out in case you didn't. You can read more about the versioning within our docs in our documentation guidelines.

[saig0]

The pages to review are under the section: https://preview.docs.camunda.cloud/pr-4270/docs/next/apis-tools/testing/getting-started/

[remcowesterhoud]

Just some small things, otherwise LGTM 🚀

[github-actions]

🧹 Preview environment for this PR has been torn down.